Maggie Sokolik, who has already had one successful MOOC (Principles of Written English on edX), has an early chapter in this online book. She is definitely worth paying attention to, and her concluding remarks in Chapter 2 are copied below:

Tips for a Successful LMOOC
To summarize the points made in this chapter, instructors and designers who are developing LMOOCs should consider the following suggestions:1. Maximize engagement and interaction
Consider the ways in which students can participate on the MOOC platform. Students will be more engaged in using language if they can communicate among themselves. The focus of learning should be in interaction, and not just in absorbing course materials.2. Facilitate, but do not manage, self-organized learning
Suggest ways that students might self-organize through social media or face-to face meetings when possible, but do not manage these groups. Students will, in fact, most likely organize these groups without instructor participation at all.3. Create an instructor presence
Students need to know that there is a human instructor participating in the
LMOOC. Take time to comment on student discussions in the MOOC, make regular announcements, create a Facebook page or Twitter feed to support the class, etc.4. Use video for engagement
Think about video as a source of authentic language materials, not just as a way to deliver a lecture. Use video as an opportunity for students to become engaged in culture as well as the language itself.5. Define success
As you develop a Language MOOC, think about what success will mean in the course. Encourage students to think of their own goals, and how they will use the course to achieve them.6. Match the goals of assessment with its form
As you think about the goals of the course, consider what type of assessment matches those goals. If the course focus is on writing, for example, then assessment types should evaluate writing in a meaningful way, such as through peer and self-assessment, rather than through multiple-choice questions.

All are very good advice, and something we need to try to incorporate in our MOOC.